1. Field of the Invention
The present invention relates to an exhaust processing apparatus adopted for, for instance, a semiconductor vapor deposition apparatus.
2. Description of the Prior Art
In growing semiconductor crystals, there are a liquid phase crystal growing technique and a vapor deposition technique. Compared to the liquid phase crystal growing technique, the vapor deposition technique has a high controllability to easily grow crystals in multilayer structure and is able to control the composition ratio of crystals according to the partial pressure ratio of source gases.
Due to these advantages, the vapor deposition technique has been developed rapidly in these days. Particularly, a metalorganic chemical vapor deposition (MOCVD) apparatus for growing thin film crystals on a semiconductor substrate is excellent to control the speed of crystal growth, easy to operate and adequate for mass production so that it is catching many attentions.
FIG. 1 is a view showing one example of the vapor deposition apparatus. In the figure, a reactor 105 has a gas inlet 101 and a gas outlet 103. A susceptor 109 is disposed inside the reactor 105 to hold and heat a semiconductor substrate 107. The susceptor 109 is supported with a rotary shaft 111 which is rotatably supported with a bottom wall 105a of the reactor 105 and rotated by a motor (not shown). The reactor 105 has a high-frequency induction heater 113 to heat the susceptor 109.
The semiconductor substrate 107 is held on the susceptor 109 and heated to a predetermined temperature with the susceptor 109 which is heated with the high-frequency induction heater 113. After that, source gases for growing crystals are introduced from the gas inlet 101 into the reactor 105. The source gases react on the semiconductor substrate 107 to grow crystals on the substrate due to the reaction and decomposition of the source gases.
If a GaAs film, for example, is to be formed on the semiconductor substrate 107, H.sub.2 is used as a carrier gas and trimethylgallium (TMG) which is organometal and arsine (AsH.sub.3) which is hydride are reacted in gas phases.
After the reaction and decomposition, the source gases which remain unreacted flow through a space 115 between a periphery 109a of the susceptor 109 and an inner wall 105b of the reactor 105 and is discharged through the gas outlet 103 to the outside of the reactor 105.
In this vapor deposition apparatus, the unreacted exhaust discharged from the gas outlet 103 includes noxious hydrides such as the arsine and organometal so that various exhaust processing apparatuses shall be installed to treat the noxious exhaust.
FIG. 2 is a view showing an example of the exhaust processing apparatuses. An exhaust processing apparatus 117 comprises a cracking furnace 119, a filter 121 and a chemical trap (a chemical adsorbing member) which are successively disposed in an exhaust flowing direction in the middle of piping 118 connected to the gas outlet 103 of the reactor 105.
The cracking furnace 119 has a heater 125 to heat the exhaust including the unreacted source gases passing through the cracking furnace 119 to crack, for instance, part of arsine contained in the exhaust into solid arsenic and hydrogen.
The solid arsenic is collected with the filter 121 disposed downstream the cracking furnace 119. Arsine which has not solidified and passed through the filter 121 is adsorbed with an adsorbing material 127 in the chemical trap 123. With the combination of the cracking furnace 119 and the filter 121, an amount of the arsine to be treated with the chemical trap 123 may be reduced to improve the service life of the chemical trap 123.
However, according to the apparatus mentioned in the above, the finer the filter 121, the higher the collecting efficiency of solidified arsenic as well as the exhaust pressure. As a result, a discharging performance is decreased and an operation at a predetermined pressure hindered. Therefore, the apparatus 117 shall be designed stronger, and sealing structures for connections of the piping 118 stricter
In addition, the arsenic solidified in the cracking furnace 119 may clog the filter 121 and piping 118. This may give adverse effects on growing crystals on the semiconductor substrate 107, or may temporarily stop the crystal growth.
Further, in such a conventional apparatus, arsenic vapor generated due to the decomposition may flow out of the cracking furnace 119 and solidify outside the cracking furnace 119 to form dusts to increase load of the dust collecting filter 121. Compared to a cross-section area of a passage of the cracking furnace 119, a diameter of the piping 118 on the downstream side of the cracking furnace 119 suddenly reduces. As a result, unreacted source gases contained in heated exhaust are cooled and solidified around a connection of the piping 118 to clog the piping 118.